The Amphetamine Phenylethylamine Effect
by wonderwoundedhearers
Summary: Penny makes it easy for Sheldon to become 'one of the idiots' in ten simple stages… Shenny: suggestive/mild.
1. Pheromones

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 1 –**

"_Pheromones"_

Dr Sheldon Cooper had never fully understood why females consciously covered their bodies in a poisonous mist of animal secretions and sickly-sweet alcoholic scents.

He looked out of the car window and up at the billboard Penny and he were passing by in her car. The advertising monstrosity could have been better used for showcasing the dates of the next Comic-Con, he thought, instead of the writhing nude bodies and twisted letters spelling '_Aphrodite_' in bright pink.

Seeking the opinion of a female to settle his confusion, he turned to Penny. Her hair was wildly flicking about in the wind coming through the open window on her side, while her mouth was shaping the words to the song on the radio.

"Penny, I find myself in need of your female insight about certain matters that I do not understand."

Penny's eyes darted off of the road and to him for a moment, before they turned back and she sighed. "I'm not going to be discussing my 'cycle' with you again, Sheldon."

"Good, because I already know that it began three days ago. My memory is faultless."

Her mouth dropped open, before she simply shook her head at herself and ignored his reply.

Seeing that she had not discouraged him from his questioning, Sheldon continued. "I wondered whether you could enlighten me as to why women insist on buying perfume."

Penny frowned at the road. "To_ smell nice_, Sheldon."

"Ah! But there is the root of my confusion! Perfumes _do not_ 'smell nice,' as you say, and they consist of a variety of ingredients, all of which seem highly ridiculous – and some most definitely unsanitary – to cover oneself in."

She shrugged her shoulders at him, already looking like she was tired of his probing. "Things smell different to different people, sweetie. Not all of us have your Spidey senses."

He gave a short nod, accepting that not every human had the honed faculties and capacities he did, and continued on another line of questioning.

"I understand the way advertising works, going for the 'weak point,' as it were, and preying on the primitive desires of _Homo sapiens_, but I wonder whether this is a fruitless endeavour as the natural pheromones released from the human body generally predict the likelihood of coitus taking place."

Penny stared at the road for a moment, wide-eyed and brain obviously thinking away, before she shot Sheldon a quick and confused glance.

"Do you _mean_…why do people use perfume when natural scents are meant to be more attractive?"

Sheldon considered her simplified sentence for a moment, before concluding that it would suffice and giving her a nod.

"Well…" She tapped her fingers on the steering wheel in thought. "I suppose…not everyone has a very attractive scent, and those people feel the need to buy a fake one to make up for it… Oh, I don't know, Sheldon. Different people like different things. Can we drop it?"

He considered her for a moment. "Do _you_ wear perfume, Penny?"

"No."

"Why not?"

"Because I think it's nicer not to. It overpowers."

Sheldon smiled, satisfied that there was at least one more sensible consumer in the world.

Penny quickly changed the subject before Sheldon could say anything more. "So, why couldn't Leonard take you home? You said he was busy – what's he up to?"

Sheldon gave a derisive snort and crossed his arms over his chest. "He's _dabbling _in interferometry for his current experiment. He's like a blindfolded monkey with a hammer – only less productive."

Penny frowned at Sheldon's insult, but he saw the twitch of her lips that indicated her desire to laugh. He postulated that it was her previous relationship with Leonard that dictated her sense of loyalty when it came to making fun of him and his research.

Their building suddenly came into view and Sheldon let out a relieved breath – the blinking check engine light on Penny's dash had been becoming harder and harder to ignore.

Penny pulled in and parked, turning off the engine and rolling up her open window, and Sheldon was suddenly assaulted by a scent.

It made him reel.

His head snapped towards Penny, who was innocently looking through her purse for the key to her apartment. Looking at her, his eyes caught a small stain on the apron of her Cheesecake Factory uniform.

"Penny…what's that?"

She looked down before rolling her eyes and throwing her hands up in the air. "Some jerk knocked me into a wall while I was carrying a tray to a table! Can you believe that? He didn't even say sorry!"

As she huffed in remembrance, Sheldon's brain seemed to…malfunction.

It was the White Chocolate Raspberry Truffle cheesecake special smudged on her apron – he could smell it – and while a part of his brain went wild at the knowledge that there was old dairy product staining her clothes, another part was completely blank because of it. That was his favourite Cheesecake Factory cheesecake.

There was another scent as well. He hadn't smelt it upon getting in the car when Penny picked him up at Caltech, most likely because she had the windows open when he got in, but in the enclosed and now-sealed space he could certainly identify it.

It was _Penny_. Pure Penny. It was fuzzy, complex, sweet and yet not _too _sweet – it reminded him of warmth and Halo nights when she would sit next to him on the couch in his and Leonard's apartment playing furiously.

He liked the smell – he liked it _a lot_.

"Sheldon? Sweetie?"

He looked up to find Penny at the passenger-side door, standing there with a confused smile on her face as she held it open for him.

"You okay?"

He paused for a moment, considering sitting in the car a moment longer to inhale the scent some more, before angrily shaking his head at himself and hurrying from the car.

"I'm fine. Car sickness. You're a terrible driver. You need to take a class. I'm going inside."

He shuffled off quickly, leaving a bemused Penny behind.


	2. Skin

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 2 –**

"_Skin"_

Eyeing the flashing inbox of his e-mail account with distaste, Sheldon knew that the new mail he had received would most probably be spam or unwanted messages from Wolowitz containing semi-explicit pictures of barely-clad women.

Either way, he had no desire to discover what _delights_ his inbox held, but the flashing was distracting him from his task of replying to Amy Farrah Fowler's latest e-mail. Not to mention, the visible differences between the inbox and the sent box beneath it were testing his 'particular' tendencies.

Abandoning his e-mail in progress, he opened up his inbox to rid himself of his unwanted mail. But upon viewing its contents he found that the new messages were neither spam nor from Wolowitz.

The sender was Penny.

* * *

><p>From: pennypenny613<p>

To: sheldoncooper0001

Received: 13:04

Subject: Sick!

_Don't knock (knock, knock) on my door unless you want a cold._

_P._

* * *

><p>From: pennypenny613<p>

To: sheldoncooper0001

Received: 14:51

Subject: Broken!

_Is Leonard in? My microwave broke. I'm hungry. I only have Doritos and spray cheese without it._

_P._

* * *

><p>From: pennypenny613<p>

To: sheldoncooper0001

Received: 16:09

Subject: ?

_Where are you? I thought your laptop was glued to your hands?_

_P._

* * *

><p>From: pennypenny613<p>

To: sheldoncooper0001

Received: 16:32

Subject: Shelly

_I didn't have anyone to talk to so I called your mom. She gave me a new recipe for pie and told me you need to call her because she has a box of old junk she thinks is yours with 'experimental laser' written on it. She said she's going to get rid of it._

_P._

* * *

><p>Sheldon gasped, flying out of his chair and through the door of the apartment without a thought.<p>

Knock, knock, knock… "Penny!" Knock, knock, knock… "Penny!" Knock, knock–

Her door opened slowly, sniffling sounds coming from behind it as the gap between it and the doorframe widened. When it was finally fully open, Sheldon remembered Penny's first e-mail about being sick.

She looked terrible. Her eyes were pink around the lids and bloodshot across the whites, and the area around her nostrils looked sore from it being wiped so much. Her hair was a little lacklustre, and her choice of clothes reflected the sickness she was feeling – the sweats and old t-shirt were far too big and hung off of her frame.

"What are you doing here, Sheldon?" She asked, though in reality it sounded much more like '_Wha' ar' ya doin' 'ere, Shel'on?_'

He immediately tucked his chin beneath the collar of his Flash t-shirt, covering his mouth and nose, as he went to take a breath. Logically, he knew that breathing through the shirt would do nothing beneficial, but as he was (unfortunately) not Vulcan he was prone to many illogical tendencies.

"Penny," his voice came muffled. "I received your numerous e-mails, the last of which disturbed me greatly. If my mother found my experimental laser, then–"

Penny just rolled her eyes at him. "She hasn't found anything, Sheldon. I lied. I did call her and she _did_ tell you to talk to her some when, but only because she hasn't heard from you in a while."

Sheldon just stared at her, wide-eyed. "You... You…_lied_?"

Penny just nodded, wincing, presumably, at his horrified expression – what she could see of it above his Flash t-shirt, at least.

"_Why on earth would you do that_?"

"I was lonely."

Despite the shock and rage rooting Sheldon to the spot, he could not deny that she sounded so small and fragile in those three words.

She continued, staring at the floor. "I thought if I told you your mom was messing with your science stuff you'd come over and bother me."

"Well…it worked," he stated.

Penny gave him a small, apologetic smile. "I know. I'm sorry, Sheldon. It was stupid. You can go on back now."

He stood there for a moment more, not saying a word as his fingers tightened their grip on the collar of his raised shirt.

Penny bit her bottom lip anxiously at his silence.

His eyes automatically flickered down to the pink piece of flesh caught between her white teeth, and he felt _something_ shudder through him. Just like her special scent – the one he could _just_ _about_ catch through his shirt – her mouth made his brain…pause.

Sheldon hadn't ever considered Penny's lips before. They were aesthetically pleasing, he supposed – both classically and generally – and the colour the bottom one turned as her teeth released it created a warmth inside of him. It was a sort of pink colour, but darker than the word alluded to – it was almost red but not quite, more like caught between the two shades.

He had previously wondered – like he had with perfumes – why women bothered wearing lipstick, or other such applications, when there was no obvious reason for doing so except to conform to a socially accepted practice and standard of 'beauty.'

But in the back of his mind, as he watched Penny's lips _pout_ slightly, he considered the thought that perhaps they were trying to accomplish something few women could achieve: naturally beautiful lips.

Penny was one of those women. Even when sick, her mouth looked so–

"Sheldon? Are you staying or going? I'm getting cold with the door open."

He shook his head free of his thoughts to see Penny with her arms wrapped around herself as if to hold onto any warmth she had retained before opening the door to him.

A curious idea crossed his mind as he looked at her for a moment longer, one which his brain was more than happy to comply with in its fuzzy state.

He immediately crossed the hall to his and Leonard's apartment and reached for his flu mask in the top drawer of his desk. He slipped it on before closing the apartment door behind him and going back to a stunned-looking Penny.

"Wha– "

Yes, he didn't quite know what he was doing either.

Sheldon pinned her with a cool look, his voice muffled by the mask. "You said your microwave was broken. As Leonard has decided to go into work on a Saturday and you are ill, I suppose I must take up the mantle of doing what society dictates as being a 'good neighbour'."

Penny dazedly stepped away from her door in silent invitation and Sheldon entered to see her apartment looking like a dumping ground.

He sighed to himself, glancing at Penny's full bottom lip once more before striding over to the (decidedly battered-looking) upturned microwave and wondering how that little bit of colour in her skin had him acting so out of sorts.


	3. Movement

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 3 –**

"_Movement"_

Sheldon wasn't really one to take much notice of trivial things. Of course, he saw them – he rarely missed anything – but, generally, he found them mundane and unworthy of his interest.

Penny had changed that, he found.

He knew that his finger and hand movements occupied a large area of his brain – as it did for every human being – and that area was dedicated to monitoring even the most minute of touches, and he also knew that his lip and tongue movements occupied the exact same size area. But he hadn't ever thought that, perhaps, there might be exactly the same facilities for monitoring _others_' movements – especially that of the hands and fingers…and the lips and tongue…

And especially those that belonged to Penny.

She was telling Leonard a joke, smiling and licking a drop of remaining tangerine chicken sauce from the smooth pale wood of one of her chopsticks, and Sheldon was mesmerised.

He had never found another person's eating habits even remotely…attractive before.

He had simply been eating his meal, Leonard droning on about his failing experiment to anyone who would listen, when, out of the corner of his eye, his brain registered something as 'interesting'. He had looked up to see Penny twisting a few curls of noodles from her chopsticks above her open and waiting mouth.

Sheldon had watched as she turned her wrist, bones shifting delicately as she twirled the noodles tightly around the chopsticks, before her light pink tongue had lightly wet her dark pink lips and the noodles had disappeared inside her mouth.

He had been stunned by the fizzing warmth he had felt deep in his stomach.

"Sheldon, sweetie, what do you think?"

He thought he might need immediate psychoanalytical testing.

Sheldon looked up. "About what?"

Penny was watching him curiously, head akilter and chopsticks down. "We were talking about all going to see the new _Planet of the Apes_ movie. Are you feeling alright? You look a little flushed."

He shook his head, not understanding one bit the way he liked Penny's lips shining slightly from the oil in the tangerine chicken sauce.

"I'm…going to go lie down."

He rushed to the safety of his room, discarding his suddenly-bland meal on the kitchen counter on the way. He could feel surprised eyes on his back at his uncharacteristic exit.

Sheldon sat on the edge of his bed, his mind whirring through the immediate possibilities of his sudden illness. It seemed that Penny was the catalyst, but he'd be damned if he knew what was causing the fluttering he felt…or the heat.

There was a sudden knock at his door – well, three of them, to be precise.

"Sheldon? Can I come in?"

Penny was the only person allowed to see the inside of his bedroom, and he wondered if it would be a little strange for him to tell her she wasn't allowed in when she had entered before.

"Come in."

His voice was strange and croaky – maybe he really _was_ sick.

His door opened slowly and Penny poked her head inside, smiling sympathetically. "You okay?"

Sheldon made no move to answer her, mesmerised as he was by the way her lips curled upwards at the corners. He came out of his trance when he heard the door shut and flinched when Penny pulled him back onto his bed.

"Come on," she murmured, and he dazedly fell back fully onto his Batman sheets.

"Wh-what are you doing?"

Penny sat on the edge of the bed, hand on his sternum over his Green Lantern t-shirt. "I was going to sing to you. You're ill right?"

He realised that she was planning on singing 'Soft Kitty,' and while one part of him knew he wasn't really sick enough to have it sung to him, the rest of him couldn't get his head around the way her hand on his chest made him feel. He knew he was flushed again.

She smiled and began to sing once she realised he had no objections. "Soft kitty, warm kitty, little ball of fur…"

Her hand circled, her palm creating friction between the skin of his chest and his shirt, and his brain paused. It was becoming increasingly frequent for his mind to freeze around Penny these days.

Sheldon felt something…stir.

"Happy kitty, sleepy kitty, _purr, purr, purr_…"

Penny's voice was soft, _husky_, and he watched with wide eyes as her hand made more soft circles on his chest as she began the song again. He felt that part of his mind focused on hand and finger movements switching to Penny, and–

He gasped.

Penny stopped. "Sweetie, did I catch a hair or something?"

He couldn't even shake his head. Her littlest finger was resting on his nipple and he could feel it crinkling beneath her touch. It was all so…

Despite his frequent denials, Sheldon had always been prone to human faults – one of them was the occasional illogical thought, another was the ease at which he could get sick, and another was the hardness then pressing against the fly of his plain plaid pants.

Ever since puberty he had had the problem of unexpected and unwarranted erections, but he had never felt the…_force_ that was behind that one before. He absently wondered whether it was because Penny had _caused _it, something that had never happened before.

Sheldon didn't get…aroused.

Penny was still watching him as he near-panted. "What's wrong?"

Her lips were shiny in the dim lamplight, calling for him to do…something he couldn't quite grasp.

"I'm fine," he found himself muttering, his voice low and betraying him so obviously. "I'm _well_."

Penny seemed to shake herself out of something, before half-smiling and standing to leave. "Okay. If you're sure…"

"I'm sure."

He watched her leave, his eyes on the hand that had rubbed his chest and the fingers that she was unconsciously flexing.


	4. Electricity

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 4 –**

"_Electricity"_

He blamed the entire thing on Penny.

She was sat there, one seat off of his spot, legs curled up and fingers gripping a glossy magazine. Innocent, some might say, but Sheldon knew the truth – she was the Devil in disguise, and every time she licked her index finger to turn to the next page his body just…went wild, with the force of something akin to a thermonuclear reactor going into meltdown.

She licked her fingertip and turned the page. Sheldon gripped the edge of his desk with white knuckles.

The door to the apartment suddenly opened. "Sheldon, I was looking at– Oh, hey, Penny! What are you doing here?"

_That_ was something Sheldon had also contemplated, just after Penny had walked in and sat down without a word while he had been typing up his latest theorem.

Leonard threw his keys into the designated bowl and kicked the apartment door shut behind him, grinning inanely at Penny.

The smile Penny returned Sheldon could only describe as _weak_. "Hey. Yeah, just hanging out with Sheldon."

He resisted raising his eyebrows in surprise – _that_ was what they had been doing? Not awkwardly sitting across the room from each other in silence, while he contemplated the positives of blinding himself so he didn't have to see her lick her finger _one more time_?

Leonard seemed just as shocked as he. "Really?"

Leonard looked over to Sheldon, eyebrows up above the heavy black frame of his glasses. Sheldon didn't bother confirming or denying Penny's statement. He had found recently that where Penny was concerned and Leonard tried to involve himself, Sheldon felt rather…protective, almost.

Penny just nodded at Leonard and went back to her magazine.

Leonard slowly – dazedly, really – wandered over to Sheldon, his voice low as he spoke. "You didn't tell me she was here."

Sheldon looked up at him, relieved that Leonard was blocking his view of Penny caressing her finger with her pink tongue.

"Considering you were not in the vicinity at the time Penny entered the apartment, and that you had not given me any express instructions as to alert you of her presence…I would say that my not informing you was reasonable."

Leonard just rolled his eyes. "Great. Thanks, Sheldon."

He strode off to his bedroom, muttering to himself.

Sheldon caught Penny staring at him, her eyebrows raised and a smile curling her lips.

"What?"

His voice was near croaking, and his bluntness was unusual – Penny was doing bad things to him.

She gave him a long low whistle, presumably to indicate something of note. "Why'd he run off?"

Sheldon turned in his chair, resting his elbows on the arm rests and steepling his fingers. He considered whether the truth of Leonard's obvious infatuation with her would drive her away or not, and he tried to come to terms with the small ache the thought of her exit created inside of him.

"I _believe_ that Leonard is frustrated that he was not in the apartment when you arrived, so as to hover around you the way electrons hover around the central nucleus of an atom." At Penny's 'look,' he clarified, "He sticks to you like molasses in July."

Penny's eyes suddenly glazed over. "What was that?"

He frowned at her. "What was what?"

"That…voice."

"The accent, you mean? You know I was born and raised in Texas, Penny. My mother's sayings have stuck with me over my life, along with my father's recipe for the perfect hangover tonic."

Penny stared at him, pink suffusing her cheekbones. "It just…sounded nice."

Sheldon watched her in confusion for a moment as she shifted in her seat, before turning back to his laptop and deciding to finish off his theorem before Leonard returned to pester Penny.

He didn't hear her behind him until it was too late, and as he spun around to face her, his mouth parting to utter something, he found his lips suddenly pressed to Penny's.

He gasped against her lips, freezing as her mouth slanted over his softly and gently…as if coaxing him.

Sheldon's heart slammed against his ribs, his blood racing with cool adrenaline and his skin prickling with natural electricity. He found those prickles sweeping up his body as Penny pouted her mouth against his.

Her lips were warm, he found, and soft – so soft they reminded him of the velveteen bear his meemaw had given him on his fifth birthday – and the fact that he had his mouth pressed against the place where Penny consumed nourishment and contained germ-ridden saliva didn't even register.

Suddenly, without meaning to at all, he jerked back and their lips parted with a bolt of gentle electricity.

She was panting. He was shell-shocked.

Leaving her bent at the waist, lips flushed red and eyes half-closed, Sheldon escaped to his room at a pace the Flash would have been proud of.


	5. Mouthwatering

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 5 –**

"_Mouth-watering"_

The kiss was not mentioned.

Sheldon found himself lying awake at night more than once considering asking Penny why she had pressed her mouth to his, but when, eventually, the sun rose and he saw her next his words would catch in his throat and he wouldn't be able to say anything. At all. He would just shuffle off when she greeted him.

Penny would still scavenge food from them when she could and offer him a ride if she had heard Leonard was working different hours to Sheldon, but other than that she made no effort to intrude on his time or their apartment.

It was as if, he thought, he had driven her away.

Sheldon knew Leonard was angry at him over this, because that night when Penny had left after Sheldon had run from her unexpected kiss Leonard had been deeply disappointed not to find Penny still on the couch. He had known, as if he had a sense for it, that Sheldon was to blame for her disappearance.

Leonard had demanded that Sheldon 'make up' with her.

Firstly, Dr Sheldon Cooper did _not_ 'make up' with people – unless expressly told to by his mother – as he was above such meaningless and trivial matters, and secondly…he was scared to speak to her again.

Sheldon had been enjoying a perfectly made and cut turkey-on-white sandwich when it was all decided for him.

Leonard had rushed in, Leslie Winkle on his arm, and demanded the apartment to himself. "I'm invoking chapter twelve, paragraph C of the roommate agreement. Get out."

Sheldon had found himself keyless and locked out in the communal hallway in under three seconds.

Chapter twelve paragraph C of the roommate agreement stated that if the noise of the roommate engaging in coitus was too loud _not _to be overheard then that roommate must find another place to engage in those activities. However, this was null and void in the case of a one-way exchanging of gifts within the week previous to the coitus taking place.

Leonard had given Sheldon a limited edition of _The Amazing Spider-Man_ two days previous. Sheldon _had_ considered it an altruistic gift, but now realised Leonard had been planning the night of noisy coitus with Winkle for some time.

Sheldon absently wondered whether he had found a worthy opponent in Leonard.

He eyed the door to their apartment shrewdly. "I have taught him well."

It was at that moment that Penny appeared at the top of the stairwell, dressed in her Cheesecake Factory uniform.

She smiled at him awkwardly while his heart began to pound. "Hi, Sheldon."

She immediately shuffled past, her keys out ready to open her apartment door.

Sheldon realised he had a choice. He could either hope Raj would take him in on short notice – which was unlikely, since Friday nights were Raj and Howard's Scrabble tournament nights – or he could talk to Penny and hope she would let him stay in her apartment for a few hours.

He turned. "Penny."

Penny swung around, wide-eyed, as her door opened.

Sheldon stepped towards her, ignoring the cold sweat appearing on his palms. "I want to…_apologise_, for running away the other night."

She looked stunned. "O-okay."

"I would also like to ask you whether it would be an inconvenience if…" Seeing her dazed expression and knowing it most likely meant a long-winded sentence would not be appreciated, Sheldon changed tack. "Can I stay in your apartment? I got kicked out."

Her face immediately lit up. "Sure, sweetie. Come on in."

Penny held the door open for him and he shuffled inside, gaze cast downward. He was already _ever so slightly_ regretting his decision.

The door shut quietly behind him.

"You know…you don't have to apologise, Sheldon. It was my fault, what happened, and I'm _really_ sorry I made you feel uncomfortable."

He watched as she shed her coat and purse, before slipping into her bedroom and shutting the door.

Sheldon brushed away the strewn articles of bright clothing Penny had left lying across the couch and sat, slowly. He wondered why her apology created a needling sensation in his chest.

Soon enough, Penny returned.

She slumped down onto the couch next to him dressed in grey shorts and a white tank top, with a dark bottle of wine in one hand and a smile tugging her lips. He noticed her hair was down, curling across her shoulders, just tickling her collarbone, and he wondered…

"Sheldon?"

He came out of his Penny-induced trance to see her shaking the wine bottle at him.

"No, thank you."

She shrugged and turned away to turn on the T.V. "Suit yourself."

He watched her as she became engrossed in some silly drama set in a hospital, and he couldn't quite explain the draw he felt to her. Her mouth was calling him.

"Penny?"

Her gaze turned on him. "Yeah?"

"Why did you kiss me?"

There was silence for a few moments, the only real noise coming from the T.V., but she didn't break eye-contact. He felt a thrill race through him.

"I wanted to."

She was always so straight-forward, so…Penny.

"Why did you want to?"

He was always so stubborn, and so very, very Sheldon.

A blush graced her cheeks. "Because…it's you. I wanted to kiss _you_."

Penny turned away after that, and Sheldon found himself considering whether her answer made any sense or not. But, even if it did or didn't, her words still affected him.

His hand closed around her wrist and he tugged the hand not holding the wine bottle. Penny turned, her expression expectant and just a little bit scared.

This time, he initiated contact. Sheldon kissed her.

Her lips were as soft as before and very warm, but they were also just as confusing. Her mouth was confusing, the way she gasped against his lips and drew in a short, sharp breath before pressing her mouth against his more insistently was confusing, and the way her hand came up to grip the short curl of hair at the nape of his neck was just as confusing as everything else.

The _most_ confusing part was the way Sheldon felt. The electricity was there, that prickling charge that swept through him and, seemingly, into Penny, but he also felt himself…salivating.

She tasted like wine: dark, fruity and slightly bitter, and he felt the warmth of that taste spread over his tongue.

Penny pulled away this time, her eyes dark and gleaming. "Sheldon–"

A knock on her apartment door interrupted her.

She gave a short growl before leaping up and going to the door to throw it open.

There Leslie Winkle stood, eyebrows high and hip cocked as she quite rudely looked straight past Penny. "Hey, Dr Dumb-ass. Leonard and I are done. You can go in now."

She gave Penny a derisory smile before striding away down the stairwell.

Penny slowly shut the door. Sheldon stood.

"Thank you for your hospitality, Penny," he murmured, moving towards the door.

Penny spun around. "You're not staying?"

He shook his head, looking anywhere but at her and suddenly so very painfully shy. He wanted to kiss her again. His courage had fled, he was scared.

She opened the door. "Go on, then. Run away."

He did just that, shooting quickly across the hall, but he didn't miss the sad sniffle Penny gave as she closed off her apartment to him with a slam of the door.


	6. Hormones

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 6 –**

"_Hormones"_

Sheldon was certain.

It had taken him three days to work out that he hadn't really wanted to leave Penny's apartment after their kiss, two more days to work out that he had in fact wanted to stay and kiss her again, and one more day to work out that she was less receptive of him because of his cowardly escape.

But that night, Saturday night, was laundry night, and he knew Penny would be in the laundry room when he went down at 8:15. She was always there when he was it seemed.

Carrying his basket down, he smelt her before he saw her.

Sweet warmth washed over Sheldon as he slipped inside the laundry room and carefully shut the door behind him.

Penny was bent over a washer, pulling out wet articles of clothing, and Sheldon had to ignore the rush of heat that ran through him as she bent over particularly far.

She didn't look up. He coughed pointedly.

Penny gasped, straightening up and spinning around. "Sheldon!"

He inclined his head. "I scared you."

"Of course you did," Penny hissed sharply, before taking a deep breath and slumping back against the washer behind her. "What do you want, Sheldon?"

He deposited his basket on the table and took a step towards her, just as something warm and bubbling ran through his system. It felt good – _empowering_.

"I want to apologise."

"_Again_?" She stared. "Look, there's no point in you apologising if you're just going to run away again. It doesn't matter. Just leave it alone."

And, with that, she turned away.

Sheldon, however, was being filled with something resembling courage, and he found himself closing in behind her.

"Penny…"

She spun around in surprise, only for Sheldon to lean in and press his mouth to hers.

He had never considered attaching himself to any one person before – the only people he cared for truly being the females in his immediate family – but as he kissed Penny, noting the adrenaline carelessly crashing through his body and the hormones taking effect, he could feel something running between them.

Sheldon had read much on biological reactions, both before and after his strange encounters with Penny, and he knew it was the oxytocin infecting his system. It was a bonding hormone of great magnitude, released in humans during situations with heightened emotion and sensation, and it only made that fuzzy ball of energy in his stomach that much more powerfully compelling.

He liked Penny. More than he cared to admit, too.

At length, Penny pulled away, eyes closed and breathless. "What was that?"

Sheldon stared down at her, confused. "A kiss."

She let out a short, amused breath, before opening her eyes and looking up at him.

"I know that, sweetie. I meant…what are we _doing_?"

Sheldon looked down at their barely touching bodies, recognising the urge to touch her, to hold her somehow, to kiss her again…and something much more dangerous…was mounting, before realising Penny meant something much less physical and more…_general_.

"I want to kiss you again," he stated.

Penny's lips twitched at the corners in a smile, before she closed her eyes and pouted her lips slightly in obvious invitation.

As much as he wanted to, he resisted the urge to accept.

His thumb brushed hers, making her take pause and look up at him again.

"I meant…another time, soon, _often_…"

His usually verbose attitude had fled, leaving him with crumbs of coherent thought and the remnants of the correct words he should have been using. But Penny seemed to understand him perfectly.

She smiled that special smile that made his brain pause – the one that made her eyes light up impossibly.

"You wanna…date?"

He considered, momentarily, whether that suitably encompassed everything he felt and everything he wanted to feel – _possibly_ – in the future. His hormone-riddled body ultimately decided for him.

"Yes."


	7. Adrenaline

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 7 –**

"_Adrenaline"_

Sheldon was not sure how he would describe the relationship he and Penny shared.

Once upon a time, he was sure he would have deemed her unworthy of wasting his vast cranial capabilities talking to her at all. She would not have been very real to him. She would not have had another purpose other than to serve customers at her place of work, no matter where that was, and pay her rent for the apartment across from his and Leonard's.

But that had changed.

He knew she wanted to be an actress. He knew her favourite colour was turquoise. He knew she worked hard at the Cheesecake Factory, earning more tips that any of the other waitresses. He knew her hair was a completely natural bright blonde. He knew she liked to paint her toenails red for the weekend, no matter who they would be seen by, if any one at all. He knew that her "guilty pleasure" was spending Sunday nights watching re-runs of _Star Trek: Enterprise_.

She was real.

He looked over her surreptitiously as she took a slice of pepperoni pizza, cut the strings of cheese connecting it to the rest of the pie, and blew a cool breath of air over it before attempting to eat any of it.

She had a system with food, something he had not recognised before they had privately agreed to 'date,' and it was surprisingly…enthralling.

Penny liked food to be at the optimum temperature – anything less was unacceptable – and anything with a solid kind of topping would be eaten carefully, with consideration. Foods that could make a mess were out unless napkins were within easy reach, and ice cream was only to be eaten if it had some kind of chocolate in it.

Refreshment was trickier to track, but Sheldon found that Penny would only drink alcoholic beverages if it was between the hours of eight p.m. and ten p.m. – the hours did not extend even if she did not have to work the next day. She liked milkshakes to be either vanilla or banana flavoured, but would only indulge herself on Saturdays. She drank the exact amount of water recommended per day, and only drank fizzy beverages if there weren't any hot ones to be had.

He had noted this all within two weeks. Her patterns were clear-cut, obviously set into a routine she had carved herself over many years, but he did not find them boring in the least.

Sheldon found that Penny's unexpected intricacies only heightened his desire for her, and on more than one occasion after indulging, as they were right then, in a communal meal he found he had to slip across the hall to her apartment and kiss her. Even if it was only once, and only for a moment…

His need for her disturbed him greatly, but nowhere near enough for him to stop seeing her.

She created the rushing sensations in his body when he was near her doing the most mundane and simple of things. She sent adrenaline coursing through him with barely a look.

Penny broke him from his thoughts, standing from her seat next to him. "Well, thanks for the pizza, guys. But I better get back."

The disappointment Sheldon felt as he watched her slip out of the apartment with a small smile and wave back was only tempered by the fact that he knew she was waiting for him to walk across the hall and knock three times on her door.

He was just about to stand when Leonard spoke to him.

"Sheldon…have you…you know, heard anything about Penny recently?"

Leonard was looking at him expectantly as Howard and Raj continued to demolish the pizzas.

Sheldon gave him a derisory look. "Define 'anything'."

"Well…" Leonard looked sheepish. "You pretty much never miss _anything_, and I was wondering whether you'd heard if…Penny has a current boyfriend."

Sheldon felt his left fist clench at the idea that Leonard was possibly preparing to romantically target Penny. He felt something bitter bubble up in his stomach at the thought of Leonard pursuing her, and for the first time in his life he found himself able to lie _flawlessly_.

"A day or so ago I did happen to see Penny enter her apartment with a male of considerable height and bulk."

Leonard seemed to pale a little. "Bulk? Like…fat?"

"Muscle. In fact, I would go as far to say that his arms resembled something that you might find on Bane in _Batman: Vengeance of Bane_. However, his face was far more aesthetically pleasing and he did not wear a mask."

Leonard sighed and ran a hand through his hair as Sheldon congratulated himself on not going _too _far in his description of the imaginary boyfriend.

"I s'pose that's for the best," Leonard muttered. "I'm just…gonna go to my room for a bit. I'll see you guys later."

They watched him shuffle off, and soon enough the pizza boxes were empty and Raj and Howard bade farewell too.

Sheldon was left alone, his only slice of pizza cold on the plate before him. He hadn't been able to eat with Penny sitting next to him smelling as good as she did.

He found himself throwing away the pizza and exiting the apartment before he knew it.

Knock, knock, knock… "P_-mphf_…"

She opened the door before he had even said her name, throwing her arms around his neck and muffling the word against her mouth.

He found his hands settling on her waist, feeling the beginnings of the lines that curved into her hips.

Adrenaline shot through Sheldon faster than Bane's Venom and as he pressed his lips more insistently to Penny he realised he was leading her back into her apartment and kicking the door shut behind them.

Penny pulled away slightly at the noise, looking up at Sheldon with dark eyes and flushed red lips.

The sight of the dark skin made his breath catch and before he knew it one of his hands was in her hair and pulling her mouth back to his.

For the first time, Penny moaned.

At first, he thought he might have pulled too hard, the adrenaline in his system rendering him unable to mind his strength, but when she tightened her grip on him and licked at his bottom lip he knew she was–

"_Sheldon_," she gasped against his mouth. "Let me in."

Before then they had only stuck to mild touches and closed-mouth kisses, but the way Penny's tongue was wetting her lips told him she wanted to put the pink muscle in his mouth.

He found he didn't particularly mind.

She gripped the short curl of hair at the nape of his neck and pulled his face back to hers. He opened his mouth to her assault and his mind promptly went blank.

There was nothing but the sensation of her tongue sliding along his and her nails gently scoring his neck.

Within seconds, he felt himself hardening against the soft lower part of Penny's belly.

"Penny…" He muttered. "Penny…"

She pulled away slowly, looking up at him blearily. "Yeah?"

He felt his ears beginning to burn, the adrenaline wearing off and the need to escape to his room becoming a priority.

Penny shifted against him and gasped. She looked down, eyes widening, and then, ever so slowly, looked back up.

A smile flitted across her lips, turning Sheldon's knees to Jell-O. "You really liked that, huh?"

Sheldon knew his ears were most definitely on fire, but Penny's hands on his face and in his hair kept him from escaping.

"Wanna do it again?"

He stared down at her, hands frozen on her waist and neck as he considered the options he felt he had left. There was only one that appealed.

Swallowing his anxiety, he nodded.

Penny's mouth was back on his within mere seconds, her tongue opening his lips against hers and exploring his mouth.

Sheldon's tongue flickered against hers, entirely of its own will, and when Penny rewarded him with a thrilling breathy moan he gave a proper response.

She gasped against his mouth as he slipped his tongue into hers, careful and exploratory.

What he had previously considered as unsanitary and meaningless then became his new favourite sensation.


	8. Impulses

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 8 –**

"_Impulses"_

Once upon a time, Sheldon had considered himself a supremely controlled being – it was an illusion which Penny had completely shattered within mere minutes.

When she kissed him he felt elation and desire, his thoughts became incomplete and incoherent, but when he kissed her…it was as if he had solved something inside of his own body. Something that had needed filling had been filled.

Sheldon was quickly becoming insensible and the concrete structure of his home and work life blurred into something unrecognisable, something that he had once thought only lesser beings were affected by – that something being change.

Everything began to revolve around Penny.

From her work hours to her menstrual cycle he began to order his life around hers, and, it seemed, that it wasn't entirely one-sided either. Penny would often ask him what he was planning on doing in his spare time through the week, and if he had something planned she would work around it.

Sheldon had heard stories of females being demanding when it came to time and attention, but Penny did no demanding.

He found himself wondering, on more than one occasion, whether Penny was an anomaly, and, if she was, whether that meant they were well matched because Sheldon considered himself somewhat of an anomaly too.

But the only problem Sheldon found with Penny was that he wanted her – _constantly_.

When she was near he wasn't able to take his eyes from her, even if he only kept them fixed to her knee, and if she even inadvertently brushed him he would find himself practically indisposed with lust.

She did strange and wonderful things to him, but none more so that the impulses he felt so keenly when she was around.

They were sitting on the couch in Sheldon and Leonard's apartment, each on their own laptop, when he received an e-mail.

Upon hearing the alert noise for new mail he opened his inbox, and he felt his eyebrows rise as he found the message was from Penny.

* * *

><p>From: pennypenny613<p>

To: sheldoncooper0001

Received: 20:03

Subject: (:

_Hi._

_P x_

* * *

><p>Over the past few weeks of their relationship Penny had taken to leaving a few symbolic kisses after her sign-off in e-mails and texts, even notes, and for some unfathomable reason this was one of the things that created a burning impulse ricocheting inside of Sheldon.<p>

Without a word, he closed the lid of his laptop and put it on the coffee table. Penny's computer followed his.

Within seconds his hands were in her hair and his tongue was prying open her mouth. Penny whimpered against his lips, her fingers circling his biceps.

The noises she made when he kissed her were another thing that had his control slipping and impulsiveness ruling his body.

He found his teeth sinking into her lower lip.

Penny moaned in response, shifting and throwing her leg over both of his until she was sat squarely in his lap.

They were both in his spot and Sheldon couldn't have cared less.

His fingers automatically ran down her back and sank into the soft flesh of her sides. Penny gasped at the impulsive move, her hips beginning to circle over his.

Sheldon's brain buzzed, before shutting off into quietness and leading him somewhere he hadn't been before. His hands mapped Penny's thighs as they framed his own, and every inhibition began to fall away…

The door to the apartment suddenly opened and Leonard tumbled in with Leslie Winkle wrapped around him, attacking his neck with passion.

The two couples stared at each other, and Sheldon automatically tugged down the back of Penny's t-shirt in an effort to cover up the soft warm skin that had been revealed by his own hands.

Leonard gaped at them both. "W-what?"

Penny gave a short sigh, her fingers retracting as she swept herself off of Sheldon.

She gave a simple answer. "We're together, Leonard."

Sheldon watched as Leonard's face twisted in something resembling pain, but Penny didn't seem that interested in seeing what Leonard had to say back. She simply tugged Sheldon up and led him past Leonard and Leslie, across the hall, into her apartment.

When the door shut behind them, he looked over her curiously.

"Penny… What was that?"

She gave him a small smile, reaching up for his mouth with hers. "He had to find out sooner or later, sweetie, and I didn't want to stick around there when things were just getting interesting. Besides, he has Dr Nut-job over there – he won't be pouting about us for long."

Sheldon shook his head slightly, feeling more than a little bit dazed, before his mind caught onto something she had said.

Before her mouth touched his, he asked, "What things were getting 'interesting'?"

Penny gave him nothing short of a smirk, her finger running down his vintage Batman t-shirt to the zipper of his plaid shorts…behind which was a pressing problem that had not abated, even after having been caught…_canoodling_ by Leonard.

Sheldon sucked in a sharp breath, and as Penny's lips met his he realised that the danger he had sensed previously was about to become a reality.


	9. Dopamine

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 9 –**

"_Dopamine"_

Sheldon had never considered coitus as anything but a means to an end – that end being offspring. But then there was _in vitro_ fertilisation, which just took all the mess and unsanitary acts out of the entire process, and he had never really fully understood the point of going through with coitus when it could be avoided entirely.

Yes, some more primitive beings found in necessary for them to feel pleasure, but Sheldon had never been one of those beings. He took pleasure out of playing Halo, or ridiculing Howard for being an engineer. He did not take pleasure in subjecting himself to nerve-wracking, possibly humiliating, and most definitely unsanitary acts.

So when Penny's forefinger curled around the button above the zipper on his shorts, popping it open with a noise that made him flinch, he was torn between clutching to his lifelong fundamentals or letting go to feel some more of those pleasant fuzzy sensations that Penny offered.

"Penny…"

She looked up, her hands pausing at his serious tone.

"Penny…" He sucked in a sharp breath. "I don't…I don't think I can…"

Her face coloured with realisation. "Oh, Sheldon… You don't want this, do you?"

His brain ticked over as much as it could in his current state.

"It's not that."

It wasn't. Sheldon suddenly found that his fundamentals…well, they didn't mean that much when it came to Penny.

"I want…" He stuttered for a moment, before reaching down and gripping one of her hands. "I _do _want this. I just…I don't…"

Penny suddenly smiled in realisation. "You don't know how. I can teach you."

She didn't continue to undress him as he thought she might, she just lifted herself up and kissed him softly.

The first thing he found himself doing was breathing in against her mouth, taking in her warm natural scent and the clean smell of her detergent coming off of her clothes. It sent tingles rushing wildly up and down his spine.

The second thing he found himself doing was running the very tips of his fingers across the small sliver of skin between the bottom of her t-shirt and the top of her jeans. Its softness had his grip tightening on her.

The third thing he found himself doing was avidly taking in every twitch her fingers made and revelling in the pressure of her mouth on his. It had his tongue joining hers, sliding sensuously into her mouth.

The fourth thing he found himself doing was absorbing the electric shocks Penny sent him through her lips to his. It made a pulsing coil emerge low in his belly.

The fifth thing he found himself doing was having his mouth nearly watering as he tasted her tongue on his, enjoying the sweet tang of strawberry still on her lips from dessert. It had that familiar pressure pressing hard against his zipper.

The sixth thing he found himself doing was relaxing as the oxytocin weaved its way through his system. It made him want Penny even more fiercely, and it created a fuzzy feeling behind his sternum.

The seventh thing he found himself doing was making noises against Penny's mouth as adrenaline crashed through his body. It had Penny responding to his sounds, crushing herself to him and echoing his soft moans.

The eighth thing he found himself doing was letting his impulses rule his body. It had him walking her back to her open bedroom door, as he slipped the very tips of his fingers beneath the back of the waistband of Penny's jeans.

The ninth thing he found himself doing was absently cataloguing the obviously copious amounts of dopamine increasing his desire for Penny. It had him incoherent and pliant against Penny's suddenly forceful attacks, craving anything she was willing to give him.

"Stop thinking, Sheldon," she breathed into his ear, her lips barely brushing over the lobe as her hands travelled back down to the zipper of his shorts. "Stop being about physics, and join me in the biology department."

Her smirk was electric. He kicked the bedroom door shut behind them.


	10. Amphetamine Phenylethylamine

_**The Amphetamine Phenylethylamine Effect**_

– **Stage 10 –**

"_Amphetamine Phenylethylamine"_

Sheldon absently wondered whether he had ever enjoyed something as much as he had enjoyed Penny.

There had been that time when he was eight and he had managed to get his first collector's edition comic book in mint condition. There had been another time when he was thirteen and his meemaw had managed to sneak him some homemade cookies when his mother had sent him to bed with no dinner. There had been…no other time that meant as much to him as she did.

Sheldon lay on his back, Penny's head on his chest with her arm across his stomach, with a sheet carelessly strewn over them both. His eyes were fixed on a small fracture in the white paint on her bedroom ceiling – he found himself caring that there was an imperfection in the room, but not because of his 'tendencies'.

He cared because he didn't want Penny's ceiling to peel and her to have to go to the trouble of getting it all re-painted. He cared because of the trouble it would cause her.

He looked down at her, her face only just visible under her spilling golden curls and slack with sleep. He felt so much for her, and definitely something in particular.

Sheldon had read many theses, but there was one in particular that struck him at that moment: the Amphetamine Phenylethylamine Effect.

Was what he felt for Penny purely biological – the effect of natural hormones and drugs in his system?

He was certain that the answer for that was in the negative, and that his biological functions were just helping along his mind.

Because he liked Penny. He liked the way she knew how to work his _Battlestar Galactica _toaster and that she could get the Cylon image directly in the centre of the toast. He liked the way she didn't pretend she knew what he was talking about when he spoke about his research, and, _really_, he liked the way he could teach her physics. He loved the way she didn't back down, even when she irritated him, and he loved the way she didn't hold anything against him. He loved the way she accepted him, and the way she took care of him and his nervousness during coitus.

Sheldon ran his hand up her back from its place on the soft curve of her waist, pulling back a few stray curls of hair from her face so he could see her better. She shifted into him, humming quietly before falling back into sleep until her eyelids flickered softly.

He loved Penny.

_Homo novus_ be damned, he had finally become one of the idiots. Amphetamine phenylethylamine was a beautiful thing.

_**Finis**_


End file.
